Lewis Carroll published two answers to this puzzle that contradicted each other
A bag holds one counter, either black or white with equal chance — you don't know which. You drop in a white counter, shake the bag, then draw one out at random: it's white. What's the probability that the counter still inside the bag is also white?
Reveal the answer
2/3, not 1/2. Before drawing, three equally likely scenarios lead to drawing a white counter: the original was white and you drew it, the original was white and you drew the one you added, or the original was black and you drew the one you added — and in two of those three the counter left behind is white. Lewis Carroll posed this in his 1893 'Pillow Problems' and notoriously published two conflicting solutions to his own puzzle.